An SG tube in an U-like form and used for a steam generator and a heat exchanger such as a feed water heater, which are used in a nuclear power plant, is produced by bending a heat transfer tube having a small diameter and a longer length into the shape of a letter U. In this SG tube in an U-like form, an inspection for detecting a flaw from the inner surface of the tube by an inner probe type eddy current test is performed as a pre-service inspection after the tube is incorporated into the heat exchanger and as an in-service after servicing for a predetermined period. An inspection standard for the inner probe type eddy current flaw detection of the tube is extremely strict because the safety of the nuclear power generation plant needs to be secured.
FIG. 1 is an example of a chart showing the result of the eddy current flaw detection of an inner surface of a tube. As shown in the drawing, in the chart are shown a signal S from a standard notch specified by Inspection Standard and a signal N having a constant cycle P. The signal N is referred to as base noises and is caused by a minute dimensional variation generated along an axial direction of the tube. The magnitude of the signal N needs to be made as small as possible so as to prevent the signal N from being falsely determined as a signal due to a detected flaw and to perform a quick flaw interrogation to thereby improve inspection efficiency. In the following description, a ratio of the signal S caused by a standard notch to the signal N is referred to as “an S/N ratio.”
For example, in the case where when the inner probe type eddy current test is performed for the inspection of the inner surface of tube, an automatic flaw interrogation is made on the basis of signals shown on the chart, when base noises are high, that is, the S/N ratio is small, a signal exhibiting a small but deleterious defect is hidden under base noises, which makes it difficult to distinguish the small deleterious defect from the base noises.
For this reason, when the eddy current flaw detection is performed, an examiner visually observes the result of the eddy current flaw detection and when the examiner finds a doubtful signal that might be generated at a specific portion, the examiner again inspects the specific portion at a lower speed to thereby distinguish the small deleterious defect from the base noises, which decreases inspection efficiency. Since the base noises are caused by a minute dimensional variation generated along a longitudinal direction of an SG tube, the reduction in the dimensional variation along a longitudinal direction of the SG tube is important so as to improve the inspection efficiency in the eddy current flaw detection.
In general, the SG tube like this is produced by a production process including the following steps of:
(1) finishing a tube into a predetermined size in a cold working process;
(2) removing the residual stress of the tube and homogenizing the microstructure of the tube in a solid solution heat treatment process; and
(3) straightening bends and out-of-roundness of the tube that are generated by the residual stress attributable to the solid solution heat treatment process, by use of a roll straightening machine in a straightening process.
In the cold working process, a cold rolling method (Pilger rolling) by a Pilger mill using rolls and a mandrel or a drawing work using tools such as a die and a plug is employed. In this drawing work, in order to reduce friction caused when the tool is brought into contact with a tube as workpiece to thereby prevent seizing and vibration/chattering from being caused, in general, a chemical treatment lubricating coating is formed on the inner surface and the outer surface of the tube to be drawn to thereby apply a lubricating treatment to the inner surface and the outer surface of the tube.
However, since the SG tube has a small diameter and a longer length, the formation of the chemical treatment lubricating coating requires a long time and a large amount of man-hours and a chemical agent used for the formation of the chemical treatment lubricating coating is comparatively expensive, which results in increasing an operating cost. Further, since an Ni-based alloy is used for the SG tube in many cases, the alloy is inhibitive for the chemical treatment lubricating coating to be formed on the surface of the alloy. Thus, in the case where the SG tube made of the Ni-based alloy is produced, the operating cost required for forming the chemical treatment lubricating coating is further increased.
Thus, in the drawing work for producing the SG tube made of the Ni-based alloy, a high-pressure drawing (forcibly lubricating drawing) is used in many cases. The high-pressure drawing is a kind of cold drawing in which a lubricating treatment is performed by a direct oil lubrication. The high-pressure drawing can stabilize the cold drawing and has a remarkable effect on the improvement of the quality of the drawn tube.
The drawing work of the tube by the high-pressure drawing is performed by the following steps of:
(1) filling a high-pressure container, into which a tube as workpiece is inserted, with a lubricating oil, and then pressurizing the lubricating oil by a pressure booster;
(2) forming a lubricating oil film between the tube and tools, i.e., a die and a plug, with the pressurized lubricating oil, the die being mounted in a leaktight manner onto an open end of the high-pressure container, the plug being securely disposed at a working position by the pressurized lubricating oil; and(3) drawing the tube in a state where the inner surface and the outer surface of the tube are forcibly lubricated by the formed lubricating oil film to finish the tube into a predetermined size by the tools.
As for the drawing work by such a high-pressure drawing, there have been proposed various methods. For example, there is proposed Patent Literature 1. In Patent Literature 1 is proposed a method for producing a tube having a small diameter and a longer length by the cold working using the high-pressure drawing, that is, a method for drawing a metal tube in which at least the last cold working including a wall thinning working is performed by a plug drawing using a high-pressure lubricating oil having a pressure of 500 kgf/cm2 or more. In Patent Literature 1, it is described that since at least the last cold working including the wall thinning working is performed by the high-pressure drawing using the high-pressure lubricating oil, the produced metal tube does not cause seizing and hence can reduce a dimensional variation along an axial direction of the tube.
In Patent Literature 1, it is described that according to a method for drawing a metal tube, a dimensional variation along an axial direction of the produced metal tube can be reduced and hence noises generated by the dimensional variation in the metal tube can be prevented in the inner probe type eddy current flaw detection of the inner surface of the tube and hence a defect on the inner surface of the tube can be correctly detected on the basis of the output of a flaw detection device. However, a surface roughness RMAX (JIS 0601) of the inner surface of the tube, which is shown by an example of Patent Literature 1, is 2.8 to 4.0 μm and an S/N ratio is 13 to 18. These values are measured before the tube is straightened by a roll straightening machine, but after straightening, it is presumed that the surface roughness and the S/N ratio of the straightened metal tube should become smaller than these values.
On the other hand, an inclined roll type system in which a plurality of concave globoidal drum typed rolls are combined is generally employed as the configuration of a roll straightening machine used in a straightening process in producing an SG tube. The inclined roll type straightening machine includes various configurations in terms of the combination of the number of rolls, the alignment of the rolls (upper and lower direction, left and right direction), and the arrangement of the rolls (cross/opposite arrangement, zigzag arrangement). However, a roll straightening machine having the rolls arranged in a crossing manner as being opposite to each other is employed in a finishing process of the SG tube.
FIG. 2 is an illustration depicting a roll alignment example of an inclined roll type straightening machine. The roll straightening machine has a plurality of pairs of straightening rolls Ra, Rb (these rolls are collectively referred to as “R”) arranged opposite to each other in a vertical direction in the state where rotating shafts cross each other. In the roll alignment shown in the drawing, three pairs of straightening rolls including entrance rolls Ra1, Rb1, center rolls Ra2, Rb2, and delivery rolls Ra3, Rb3 are arranged opposite to each other and an auxiliary roll Rc is arranged at the delivery side of the delivery rolls. A roll straightening machine having a roll alignment like this is usually referred to as a (2-2-2-1) type straightening machine.
A gap between opposite rolls and a cross angle of a pair of rolls Ra1, Rb1 can be individually adjusted. Further, vertical positions of paired straightening rolls Ra1, Rb1 and next paired rolls Ra2, Rb2 can also be individually adjusted. Yet further, a horizontal interval between paired straightening rolls Ra1, Rb1 and next paired rolls Ra2, Rb2, that is, a stand interval can also be individually adjusted.
When bends of the tube are straightened, a cross angle θ of the rotating shafts of the respective straightening rolls R to the tube to be straightened, that is, a roll angle is adjusted in such a way that the surface of the tube 1 to be straightened is along the surfaces of the straightening rolls. Further, the gap of opposite paired straightening rolls Ra1, Rb1 is set slightly smaller than the outside diameter of the tube 1 to be straightened to thereby apply crushing to the tube 1 to be straightened and the crush height of the straightening rolls Ra2, Rb2 arranged next to the straightening rolls Ra1, Rb1 is adjusted to thereby apply offsetting to the tube 1 to be straightened, whereby the bends and out-of-roundness of the tube 1 to be straightened can be straightened.
As for the method for straightening a tube by a roll straightening machine, there have been also proposed various methods. For example, there are proposed Patent Literatures 2 and 3. In Patent Literature 2 is proposed a method for straightening a tube by which an inspection of the tube can be performed at a high S/N ratio in the inner probe type eddy current flaw detection of the inner surface of the tube by the use of the straightening rolls in which at least an outside surface layer of a roll body is formed of an elastic member having a hardness Hs of 50 to 100 measured by a spring hardness test (A type) specified by JIS K 6301.
In an example in Patent Literature 2, a (2-2-2-1) type straightening machine is used as a roll straightening machine and an offset amount is set at a large amount of 10 to 11 mm. Moreover, in the example of Patent Literature 2, a variation in the outside size of a produced SG tube is shown and is 0.004 to 0.005 mm. However, a level of stress developed on the outer surface of tube by the cold working and the straightening is different from the case on an inner surface and hence a dimensional variation on the outer surface along a longitudinal direction of the tube is also different from the case on the inner surface. Thus, even if the tube is straightened by the roll straightening machine described in Patent Literature 2, it is not clear whether or not the dimensional variation along a longitudinal direction of the inner surface of the tube can be deterred. Further, the S/N ratio of the SG tube shown in embodiment examples of Patent Literature 2 is as low as 20 to 50.
According to a method for straightening a tube described in Patent Literature 3, a tube is straightened by at least three pairs of straightening rolls, each pair of rolls being arranged opposite to each other, that are disposed on a delivery side by applying offsetting to the tube, the offsetting being formed by three positions along a tube axial centerline, each position being a crossing position of upper and lower straightening rolls, wherein η specified by Formula (1) described below is set at 1.0×10−3 to 1.5×10−3.η=(1/R)×(d/2)  (1)where given that d (mm) denotes an outside diameter of the tube, L (mm) denotes a stand interval of the roll straightening machine and δ (mm) denotes an offset amount, R=(δ2+L2)/2δ is satisfied.
In Patent Literature 3, it is described that according to a method for straightening a tube, η specified by Formula (1) described above satisfies a predetermined range and hence it is possible to perform an inspection of the inner surface of the produced tube by the inner probe type eddy current flaw detection at a high S/N ratio. In an embodiment example of Patent Literature 3 is shown an S/N ratio of an SG tube which is straightened by the use of a (2-2-2-1) type straightening machine having three pairs of straightening rolls with an offset amount of 6 mm or more applied thereto, and the value of the S/N ratio is 32 to 91. Further, in the example of Patent Literature 3, a dimensional variation in the inner surface of the SG tube is not addressed.
When the SG tube is produced, bends and out-of-roundness are generated in the tube by a residual stress caused in the solid solution heat treatment process, so that the bends and out-of-roundness need to be straightened in the straightening process performed thereafter. However, according to the conventional method for straightening a tube described in Patent Literatures 2 or 3, when the bends and out-of-roundness of the tube are straightened by the (2-2-2-1) type straightening machine, it happens that the dimensional variation in the inner surface of the tube should become noticeable and hence should decrease an S/N ratio in an inspection by the eddy current flaw detection to reduce an inspection efficiency in some cases.